The present invention relates to a control device for a stencil duplicating machine of the type having a stencil or master making section, printing section, paper feeding section and discharging section in an integrated assembly.
A stencil duplicating machine of the type described is extensively used since it is cost-effective in printing a document image on a great number of paper sheets. A stencil or master making section forms cuts in a master sheet in a pattern corresponding to a document image. The master sheet is then wrapped around a drum included in a printing section. While the drum is in rotation, a paper sheet fed from a paper feeding section is pressed against the master sheet by a press roller. As a result, ink is fed from the inside of the drum to the master sheet and further to the paper sheet through the cuts of the master sheet, printing out the document image on the paper sheet. The used master sheet is driven out to a discharging section, and then a new master sheet is wrapped around the drum to execute the above-mentioned iterative sequence of steps. The prerequisite is that the used master be surely discharged and the new master sheet be surely wrapped around the drum.
The problem with a conventional integrated stencil duplicating machine is that it lacks the function of determining whether or not a master sheet is present on the drum. Specifically, it is likely that the operation for discharging a used master sheet occurs in the master making process despite that the used master sheet has already been removed from the drum and discharged. Conversely, when the used master sheet remains on the drum even after the discharging operation due to some error, it cannot be detected and, hence, a master making operation occurs immediately after the discharging operation. Then, the resulted new master sheet wraps around the used master sheet existing on the drum, so that the latter prevents ink from the drum from reaching the former. Further, when the printing operation begins before a new master sheet is positioned on the drum, a paper sheet fed from the paper feeding section sticks to the drum due to the ink supplied from the drum surface and remains on the drum without being discharged. Then, the operator has to pull out the drum from the machine body and then remove the paper sheet from the drum. This not only wastes the paper sheet smeared all over by the ink but also causes the operator's hand and cloths to be stained. In addition, when the paper sheet sticks to the drum, a new master sheet produced by the next master making process will wrap around the paper sheet and, therefore, will not be supplied with the ink from the drum.
In the above-described type of machine, the image area of a master sheet in which an image pattern is to be formed is determined by the size of a document or the magnification change ratio thereof. More specifically, the image area as measured in an intended direction of paper transport, i.e., in the rotating direction of the drum depends on the document size or the magnification-changed image size. Assume that the paper sheets stacked in the paper feeding section are of the size smaller than the document size or the magnification-changed image size, and that each paper sheet is fed out in such a manner as to coincide at the leading edge thereof with the leading edge of the image area of the master sheet. Then, the image area will partly protrude from the paper sheet in a trailing edge portion thereof and will therefore be printed on the press roller to smear it. The smeared press roller in turn smears the back of paper sheets which are sequentially fed from the paper feeding section.